High-frequency series-resonant fluorescent lamp ballasts have been previously described, such as in U.S. Pat. Nos. 3,710,177 to Ward and 4,370,600 to Zansky. However, these previously described ballasts do not provide solutions to several basic problems associated with practical applications of such ballasts. These problems relate to the excessive power drain by and the self-destructive nature of the series-resonant ballast under the condition of being connected to an inoperative lamp.
In powering a fluorescent lamp by way of a high-frequency series-resonant ballast, where the ballast constitutes a high-Q resonant L-C circuit series-excited from an AC voltage source and parallel-loaded by the fluorescent lamp, there is a serious problem associated with the situation where the fluorescent lamp for one reason or another ceases to constitute an effective load for this high-Q series-excited L-C circuit. In such a situation, which is most apt to occur toward the end of normal lamp life, the power drawn by the high-Q resonant L-C circuit from its AC voltage source is so excessively high as to cause damage to or even destruction of the L-C circuit and/or the AC voltage source. If, contrary to expectations, destruction of the circuit and/or the source does not occur, the amount of power drawn from the source will be so large as to represent an unacceptable level of energy waste--recognizing that it may often take a long time before a worn-out fluorescent lamp is replaced.